1. Field of the Invention
This invention relates generally to a sintered alloy very high in wear resistance at high temperatures, and more particularly to such a high temperature wear resistant sintered alloy to be used as the material of a valve seat in an automotive vehicle engine.
2. Description of the Prior Art
Recently, automotive vehicle engines have been improved in power output and other performances and therefore operated under very severe conditions. Accordingly, a valve seat of intake and exhaust valve mechanisms is also subjected to very severe temperature and wear conditions and required to be highly resistant to such a conditions. For example, in a LP-gas fueled engine mounted on taxicabs, valve and its valve seat are used in a dried condition or without liquid fuel, and therefore the valve seat is liable to earlier wear as compared with that of a gasoline fueled engine. Additionally, in cases of engines operated on a gasoline containing a high rate of lead compound so that sludge is adhered on the valve mechanism thereby to increase bearing pressure against the valve seat or in case of diesel engines operated in high temperature and high compression ratio, high wear resistance and such high strength as to prevent deformation of valve seat are required for the material of the valve seat.
Otherwise, although lash adjusters have been put into practical use to accomplish automatic adjustment of the position of the valve and valve operating timings in the event that the valve seat has weared, problem of shortened life of engines due to valve seat wear has not yet been solved. In view of the above and eager requirement of lowering production cost of the valve, valve seat material high in wear resistance at high temperatures has been required.
In this regard, a high temperature wear resistant sintered alloy was proposed as the material suitable for valve seats of LP-gas fueld engines and diesel engines, as disclosed in Japanese Patent Provisional Publication No. 62-10244. The matrix of this sintered alloy consists essentially of nickel ranging from 0.5 to 3% by weight, molybdenum ranging from 0.5 to 3% by weight, cobalt ranging from 5.5 to 7.5% by weight, carbon ranging from 0.6 to 1.2% by weight and balance being substantially iron. A hard phase is dispersed in a suitable amount in the matrix. The hard phase is formed of an intermetallic compound consisting essentially, of molybdenum ranging from 33 to 36% by weight, silicon ranging from 4 to 12% by weight and balance cobalt, or another intermetallic compound consisting essentially of molybdenum ranging from 26 to 30% by weight, chromium ranging from 7 to 9, silicon, ranging from 1.5 to 2.5% by weight and balance being cobalt. The above-mentioned Publication also discloses that the sintered alloy is infiltrated with lead so that the pores are filled with lead in order to further improve wear resistance of the sintered alloy, and that it is effective for the lead containing gasoline fueled engine and the diesel engine to re-compact the sintered alloy material thereby making its density higher.
Although such a sintered alloy is improved in high temperature wear resistance over other conventional materials for valve seat, it is still insufficient in material strength. In other words, the strength of the sintered alloy is insufficient for the material of valve seat in the lead containing gasoline fuel engine and the diesel engine. In this regard, re-compacting of the sintered alloy may improve the strength of the material but complicates production process and prolongs production time of the material, thus raising production cost per a unit weight of the material.